Pierre Shale Research Paper

Pierre Shale

Introduction:

Millions of years ago, the earth had a different geographical look and the seas covered vast quantities of land that can now be seen. Many present day coastlines were completely submerged and the waters invaded deep into continents making vast portions of the landmasses an aquatic environment. These aquatic environments supported many different types of living creatures, from large mammals all the way down to plankton, algae, and microscopic organisms like bacteria and such. As time went on and these bodies of water subsided, they left an organic layer that covered the geographic area where it was. This organic matter, overtime, would eventually get covered up and it would form different layers of the earth. Many

of these layers contain hydrocarbons that have formed from this organic matter. Time, with the addition of heat and overburden stress, is the formula that makes this organic matter turn into the resources that are known as oil and gas.
These different time eras in the earth’s evolution can be dated back and mapped out. Different layers of the earth can tell us what geological event took place, what types of animals were present during that time era, what weather events happened, and an array of other things. Geologists use these different layers of the earth to structurally put together time frames of the earth’s history. One of these layers that are of great importance is The Cretaceous layer. The Cretaceous Period, which deposited the cretaceous layer, of the earth is studied because this is when it is assumed that the dinosaurs were still in existence and they covered the globe, roughly from about 146 million to 65 million years ago. The Cretaceous Epeiric Sea can be seen in figure 1 and this is where the basin was formed that has different rock formations in it that contain present day hydrocarbons. As The Cretaceous Period came to an end, so did most of the dinosaurs. This was considered the Cretaceous-Tertiary extinction; most people know it as the great extinction of dinosaurs [6]. With the changing of these eras, from the Cretaceous Period to the Jurassic Period, the break-up of the world continent Pangea continued and time moved on. Over time, new layers of sediment was placed on top of the Cretaceous Period sediment. These layers overtime changed due to overburden stress and chemical reactions. They form different types of rocks, and the Pierre Shale is one of these rocks that were formed.

Figure 1: Map of the Cretaceous Sea that was the basis of the Cretaceous rock formed roughly 146 million to 65 million years ago [4].

The Pierre Shale is a division of the Upper Cretaceous marine rock that was formed from the end of the Cretaceous era until present day. It can be found in Montana, South Dakota, Colorado, New Mexico, Nebraska, Wyoming, and Minnesota and is named after Fort Pierre in South Dakota where exposures were studied. The Pierre is dark gray shale that contains some percentage of sandstone and many layers of bentonite [7]. The Pierre Shale has different thicknesses associated with it. Thicknesses can range from 5,200 feet in the Kassler quadrangle to more than 8,000 feet northeast of Boulder. The Fox Hills Sandstone covers this shale layer and the Niobrara Formation can be found underneath it. The shale outcrops can be found running parallel to the Front Range in Colorado [1]. Figure 2 shows the outcrops of the Pierre shale. The Pierre shale is in many different basin settings and can be encountered while drilling in the Raton Basin, Boulder Basin, Niobrara formation, and many others.
Objection and Results:
The objective of this paper will be to explain about drilling rates and drilling problems in the Pierre Shale and how different variables can affect it. What will happen with penetration rates if different bits can be used to drill into this shale? How does different types of the drilling fluids affect the characteristics of the Pierre shale? How does moisture content affect the shale? Does Pierre shale cause bit balling in the drilling process? These are all very important aspects that need to be answered in order to drill into these troublesome shale formations.

Figure 2: Outcrop of the Pierre Shale in the Niobrara State Park (Google Images)
Shale is a rock that has been a burden to drillers ever since the oil field has come into existence. These rocks characteristics make it a very hard candidate to drill through and keep well integrity stable. Shale of all kinds have characteristics that are unique to each, and mostly all of them have changes of degree when they come into contact with moisture. These shales interact with water because of the clay content that can be observed in them [2]. Clay is the culprit that makes shales swell in the drilling process. Drilling fluids that are used by drillers to dig deep into the earth become a major concern when drilling into these shale formations. Drillers must use drilling fluids in order to clean the cuttings out of the hole, lubrication for the drill bit, and keep wellbore stable. Water-based drilling fluids were the first type of drilling fluids used. It came about when some drillers ran a herd of cows through a pond of water to muddy it up and use it to drill. This idea has grown into a much more complicated issue than what the first pioneers made it out to be. Now drillers can use oil-based mud instead of water based mud to drill holes. These oil based muds can be made to mimic wellbore formations better and help in controlling different aspects in drilling, like swelling of shales. But, with this new oil based drilling fluid technology comes other issues, environmental issues. Environmentalists do not like when oil companies dispose of these oil based drill cuttings and they push for drilling companies to drill with water based drilling fluids. Terry Hemphill, Barold Drilling fluids, and R.K. Clark, Shell Development Company, performed experiments that tested effects of PDC-bit selection and mud chemistry on drilling rates in the Pierre shale [5].
Hemphill and Clark stated, “ that the main objective of their tests was to identify a water-based with “oil-mud-like” shale drilling performance during drilling with a PDC –bit”.

They wanted to try to develop a water-based mud that would not have a strong effect on the Pierre shale and would be environmental friendly to dispose of the cuttings. They studied the PDC bits in order to help the drilling rates while drilling through this troublesome shale. They performed the project with two 8 1/2 –in PDC drill bits and nine different types of drilling fluids. Two of the drilling fluids were oil based while seven of the drilling fluids were water based. The different types of drilling fluids can be seen in figure two. The cores used for the experiment came from outcrops of the Pierre shale in Colorado and they were preserved in diesel oil in order to make the shale fully saturated during the experiment and this would model a soft formation where bit balling would be prevalent [5]. The two experimenters also used two types of PDC bits for the experiment; a steel-body ribbed PDC and a matrix-body bladed PDC bit. Hemphill and Clark ran many different tests changing the flow rate, weight on bit, drilling fluids, and the two different drill bits. They first concluded that the two different bits had different penetration rates for the Pierre shale. The matrix bodied bladed bit drilled faster than the steel body bladed bit in all of the mud samples

[5].

Hemphill and Clark also concluded that oil based muds helped to have a faster drilling rate in the Pierre shale than did the seven water-based muds.

This is prevalent in many present day drilling operations. Oil based mud can be scientifically altered so well to match the formations being drilled in, that it lessens the likely hood of bit balling or slow penetration rates. They did find that the water-based mud that had the cationic polymer in it, exhibited the highest rate of penetration of all the water-based mud in every tests

[5].
J.C.Fooks and M.B. Dusseault also studied the Pierre shale and how its strength is determined by the moisture content that is exhibited in the shale. They wanted to show how the reactive Pierre shale could cause borehole failures and problems in drilling, such as stuck pipe[3]. Once experimental results could be made for the Pierre shale, they could be used to mimic other shales that are deeper and harder to get to for experimental purposes. Fooks and Dusseault calculated the moisture content in each sample after the samples were submersed into oil-based mud, which was constantly circulated to maintain an equal distribution of fluid over the samples. After knowing the moisture contents, they were able to perform different test experiments to see what samples had a higher strength rate. In order to test the strength of the shale, they used a “strain rate of 1 mm per hour and three confining stresses of 3.45, 13.8 and 24.1 MPa (500, 2000, and 3500 psi). These testing levels are comparable with burial depths that possible geologically” [3]. After performing these strength tests, Fooks and Dusseault concluded that the strength of Pierre increased if there was a lesser moisture content in the sample. This showed that whenever drilling fluids came into contact with the shale, it is possible that the strength of the shale will be lowered and this could cause failure in well integrity or caving in on the drill pipe, which would cause a stuck pipe.
These facts of the Pierre shale are very beneficial to drillers that have to drill through the shale in order to reach formations such as the Niobrara formation. The Niobrara formation is a formation that is starting to be a big player in the U.S. oil boom. Seen in figure 5, The Niobrara is located in Colorado and extended into a few surrounding states. The Pierre shale that is discussed earlier in the paper overlies it.

Figure 5: Locations of the Niobrara Formation (Google Images).
This layering is demonstrated in figure 6. It is here where the Pierre shale causes a lot of wellbore integrity problems and can cause drillers to have bit balling or stuck pipe.

Figure 6: Different Layering Drillers must drill through to reach the Niobrara Formation (Google Images).

Conclusion:
Shale is a rock that can cause many different problems for drillers. The chemical instabilities that it portrays can cause drilling through it to be very hard and complicated tasks. The Pierre shale is a certain type that is studied thoroughly so that different drilling companies can know how to drill through it in order to recover some of the hydrocarbons that can be found in deeper formations. Studies like Hemphill and Clark, and Fooks and Dessault can provide information that can help drilling processes. By knowing what bits and drilling muds will help in increasing rate of penetration and decrease in bit balling, and also knowing how the moisture content can effect the Pierre shale, Drilling companies and operators can make choices in the drilling equipment that will help in decreasing the economics of drilling through the Pierre Shale.